Proteins are at the crossroads of virtually every biological process. The goal of this research is to probe protein structure and function using in vivo site-specifically incorporated unnatural amino acids, a technology recently pioneered by Schultz and coworkers. This study will facilitate research in all areas of biomedicine which is ultimately protein-related. In particular, in the E. coli system, more orthogonal tRNA-synthetase pairs will be found to incorporate more interesting unnatural amino acids e.g. 3-acetyl phenylalanine into proteins of interest. Applications of incorporated unnatural 3-acetyl phenylalanine as a "ketone handle" will include site-specifically protein-modification with 1)fluorescent molecules to allow the real-time visualization of dynamics of target protein in vivo, 2) biotin to detect and purify the protein of interest from cell lysate. Furthermore, a genetic selection will be developed to incorporate unnatural amino acids into proteins in mammalian cells. The success of this research may revolutionize the study of mammalian cells by directly accommodating tailored functional groups in proteins. This technology would be very useful for: 1) generating site-specific protein modification (phosphorylation, glycosylation etc.) at will - by-passing post-translational modifications. 2) generating new protein functions that nature has not created from the twenty common amino acids. These protein functions would be useful for understanding cell biology and creating reagents for therapeutic intervention.